Controlling the alignment of liquid crystal molecules is one of the most important technologies of manufacturing a liquid crystal display. The image quality of the liquid crystal display is related to the alignment of liquid crystal molecules. High-quality images can be presented only when the liquid crystal molecules are aligned steadily and uniformly in the display panel. A thin layer which is utilized for directionally arranging the liquid crystal molecules is generally called a liquid crystal alignment layer.
According to different applications and principles, liquid crystal alignment technologies can be classified into a multi-domain vertical alignment (MVA) technology and a polymer sustained alignment (PSA) technology, for example.
Taiwan Patent No. I325081 discloses a display panel utilizing the MVA technology. Please refer to FIGS. 1A and 1B. FIG. 1A is a diagram showing a pixel disposed in a traditional liquid crystal display panel utilizing the MVA technology. FIG. 1B is a sectional view of the pixel shown in FIG. 1A. As shown in FIG. 1B, the traditional MVA liquid crystal display panel includes a first substrate 12, a second substrate 14 parallel to the first substrate 12, and liquid crystal molecules 15 disposed between the first substrate 12 and the second substrate 14. As shown in FIG. 1A, the first substrate 12 has pixel areas 100 which are defined by scan lines 122 and 122′, and data lines 124 and 124′. Each pixel area 100 includes a storage capacitor bus line 126 which is parallel to the scan lines 122 and 122′, and the storage capacitor bus line 126 is set across the pixel area 100.
As shown in FIGS. 1A and 1B, the traditional MVA liquid crystal display panel utilizes bumps 125 which are disposed in the pixel 100 for aligning the liquid crystal molecules 15. The bumps 125 are arranged in different regions and the respective planes of the bumps are inclined so that the liquid crystal molecules 15 are tilted along different directions, and therefore the pixels 100 can form multiple display regions so as to accomplish a feature of wide viewing angle. However, in the traditional MVA technology, the display aperture ratio is affected by the bumps 125, resulting in decrease of the penetration rate. Moreover, the traditional MVA technology has drawbacks of dark fringes in a bright state and light leakage in a dark state, leading to degradation of the image quality.
U.S. Pat. No. 6,903,787 discloses a display panel utilizing the PSA technology. Please refer to FIGS. 2A to 2C, which are diagrams showing a flow scheme of a conventional PSA process for aligning the liquid crystal molecules with a liquid crystal alignment polymer. As shown in FIG. 2A, two parallel substrates, i.e. a first substrate 22 and a second substrate 24, are provided. A first conductive layer 221 and a second conductive layer 241 are respectively disposed on opposite surfaces of the first and second substrates 22 and 24. The first and second conductive layers 221 and 241 are coated respectively with polyimide (PI) alignment films 223 and 243, in advance. Each polyimide molecule has an imide radical which makes the main chain possessing remarkable rigidity and strong molecular interaction so that the PI alignment films 223 and 243 are able to be utilized for auxiliary alignment. Next, liquid crystal molecules 252 and a monomer material 254 are poured into a liquid crystal accommodating space 25 which is confined by the first and second substrates 22 and 24, and more specifically, located between the two PI alignment layers 223 and 243. As shown in FIG. 2B, a voltage source 261 is connected to the first conductive layer 221 on the first substrate 22 and the second conductive layer 241 on the second substrate 24. The voltage source 261 applies a voltage difference to the first and second conductive layers 221 and 241. The voltage difference makes the liquid crystal molecules 252 twisting at a pre-tilt angel. Moreover, an exposure procedure is performed with an ultraviolet light (UV) 262 to polymerize the monomer material 254. As shown in FIG. 2C, after the monomer material 254 is polymerized, polymer alignment layers 228 and 248 are respectively formed on the first substrate 22 and the second substrate 24. The polymer alignment layers 228 and 248 have a function of aligning the liquid crystal molecules 252.
In addition, Taiwan Patent Publication No. 200944901 discloses an alignment technology utilizing one monomer material to form a polymer for aligning the liquid crystal molecules. In this prior art, two procedures are adopted to expose the monomer material. In order to avoid destroying the liquid crystal molecules, rays of which wavelengths are longer than 290 nm are selected in a first exposure procedure for polymerizing a part of the monomer material to form two polymer steady alignment layers. Rays of which wavelengths lie between 290 nm and 340 nm are selected in a second exposure procedure for polymerizing the remaining monomer material. The '901 TW published patent is capable of solving the problem of poor performance of image sticking test for the liquid crystal display panel caused by a great residual amount of the monomer material.
Compared to the MVA technology, the PSA technology can make the liquid crystal molecules aligned much steadily. Also, bump structures are not required in the PSA technology, and therefore the problems of dark fringes in a bright state and light leakage in a dark state do not exist. Therefore, the PSA technology is capable of increasing the penetration rate for the display panel and decreasing brightness for the dark state.
However, the liquid crystal alignment process disclosed by the '787 US patent needs to coat the PI alignment films in advance. A PI coating generally at least requires equipments such as a coating machine, a baking machine, a heating plate, and a cooling plate. The PI coating results in requirement of expensive equipments so that the manufacturing cost is hard to be cut down and reduced. Moreover, the PI coating needs to use a PI cleaner, and high-temperature gas exhausted from the process will cause a great harm to the environment. Furthermore, the liquid crystal alignment process disclosed by the '901 TW published patent adopts two exposure procedures to polymerize the monomer material, in which two UV tubes of different spectrums, together with filters, are respectively used in the two procedures. Therefore, the time for manufacturing products is lengthened and the whole manufacturing cost will be increased.
Therefore, how to solve the problems of high costs and waste pollution caused by coating the PI alignment films in advance in the conventional PSA technology, and how to decrease the manufacturing cost due to exposing the monomer material by multiple procedures are important issues in this technical field.